The present invention relates to a semiconductor on insulator substrate, and more particularly to an input/output protection circuit for a MOS type integrated circuit formed in a silicon single crystal thin film on an insulation layer, and also relates to a light valve device using the semiconductor device.
Heretofore, an input/output protection circuit for an integrated circuit formed in a silicon single crystal thin film on an insulation layer has been generally formed in a silicon single crystal thin film device forming layer.
As shown in FIG. 2, since a conventional input/output protection circuit 104 for an integrated circuit on a silicon single crystal thin film 103 on an insulation layer 102 has been formed in the silicon single crystal thin film device forming layer 103 having the same thickness as a MOS type integrated circuit forming region 105 requiring high speed operation, there has been a problem in that a diode or a field transistor used as a device for the input/output protection circuit cannot realize sufficient function compared with the case of forming such a device on a bulk silicon substrate. This is attributable to the fact that the thickness of the silicon single crystal thin film device forming layer must be greatly reduced (less than 100 nm such that the channel region of the MOS type transistor is completely depleted in order to provide the high speed characteristic of the MOS type integrated circuit on the silicon single crystal thin film on the insulation layer and, if a normal diode or field transistor is formed on such a thin film, the junction portion of P- and N-type diffusion regions 106, 107 is restricted to the lateral side thereof in the case where the diode is formed as shown in FIG. 4, greatly reducing the junction area. Also, in the case of a field transistor, a field oxide film 110 reaches as far as a buried insulation film 102, as shown in FIG. 6, which results in the field transistor being unable to form.
Further, as shown in FIG. 13, in an active matrix liquid crystal display device in which a silicon single crystal thin film 103 is disposed on an insulation layer 102, a driving substrate 501 formed with a group of integrated circuits 502, 502' and a counter substrate 504 formed with a counter electrode 503 are disposed on an insulating substrate 102' film, the driving substrate and the counter substrate being bonded together with a gap therebetween and with a liquid crystal 505 disposed in the gap, a light valve device has been known in which the silicon single crystal thin film device region includes field effect thin film transistors 506 arranged in a matrix for selectively supplying a voltage to the liquid crystal layer 505 and driving circuits 502, 502', including MOS type thin film transistors for driving the field effect thin film transistors, and has an input terminal portion 507 for inputting signals from the outside to the driving circuit.
The light valve device can incorporate a driving circuit formed by single crystal silicon LSI technology and has a feature capable of reducing the number of connection terminals to the outside and having high precision and microscopic size. However, in the manufacturing step for the liquid crystal display device, since, for example, the semiconductor device is subjected to high speed rotation or cloth rubbing for the orienting treatment of liquid crystals in the manufacturing stage for the liquid crystal device, there has been a problem in that Electro Static Charge generated during the manufacturing stage leading to frequent inferiorities, and Electro Static Charge in the atmosphere during manufacture and where the finished product is placed intrudes into an input terminal portion 507 where a conductive film is exposed, destroying the device.
It is an object of the present invention to overcome the foregoing problem and provide a MOS integrated circuit formed in a silicon single crystal thin film on an insulation layer having an input/output protection circuit capable of providing sufficient function and a light valve device using the integrated circuit and provided with a sufficient countermeasure for Electro Static Charge.